Evidence of terrestrial discharge of deep groundwater on the Canadian Shield from helium in soil gases

Assessment of the impact of deep geological disposal of nuclear fuel wastes at a site in the Canadian Shield requires knowledge of the location and size of areas of discharge of deep groundwater from the vicinity of the underground disposal vault. A strong He anomaly has been detected in soil gases in a 10 X 10 m area of wetland on the banks of Boggy Creek, near Lac du Bonnet, Manitoba. The area has He concentrations in near-surface soils as high as 360 nL[center dot]L[sup [minus]1] and is assumed to indicate discharge of He-rich groundwater through a permeable subsurface bedrock fracture. Elevated Cl[sup [minus]] concentrations in groundwater and its use as a [open quotes]deer lick[close quotes] support this interpretation. A He flux density of [approximately] 2.1 L[center dot]m[sup [minus]2][center dot]a[sup [minus]1] is determined from a depth profile of He concentrations at one location in the site. A total He flux of 270 L[center dot]a[sup [minus]1] is determined for the entire site, which corresponds to a deep groundwater discharge of about 26 000 L[center dot]a[sup [minus]1]. This estimate is comparable with He fluxes and calculated groundwater discharges for two other lake-bottom locations on the Canadian Shield. 26 refs., 6 figs., 1more » tab.« less